1. Technical Field
This invention pertains to a specialized tube bending mandrel having ease of assembly and durability.
2. Background Art
Tube bending mandrels are typically used to support the inside of a tube as it is being bent. The mandrel is inserted into the section of a tube that is to be bent. As pressure is applied to the tube to bend it into a desired shape, the flexible mandrel bends with the tube but supports the inside of the tube to prevent collapse or undue distortion. After the tube is bent to the desired shape, the mandrel is removed from inside the tube. Specific tube bending practices are taught in U.S. Pat. No. 3,118,488 to Barnhill and U.S. Pat. No. 3,456,482 to Maier et al.
Commonly used mandrels are constructed of articulated links connected in a flexible line. Each link typically includes a ball end and a socket end, with each socket end being shaped to fit snugly over the ball end of an adjacent link. (Alternative designs are revealed in U.S. Pat. No. 1,683,573 to Mueller et al. and U.S. Pat. No. 4,493,203 to Wheeler et al., utilizing mandrel designs without ball and socket links.) When the mandrel is inserted into a tube, the interior of the tube contacts the outer surface of ball segments surrounding these articulated links. Pressure is applied to bend the tube to a desired shape. As pressure is applied to bend the tube, the ball of each link rotates within the socket of the adjacent link, allowing the mandrel to move with the tube, while providing interior support to the tube to avoid the tube being crushed. See e.g., U.S. Pat. No. 1,978,452 to Flodin, U.S. Pat. No. 3,415,107 to Ruscitti, and U.S. Pat. No. 3,456,482 to Maier et al.
Each mandrel link may be constructed as a single unit ball and socket, as shown in U.S. Pat. No. 3,455,142 to Roberts and UK Patent 2 229 492. Other mandrel designs utilize a link constructed in multiple sections, to provide more flexibility within the mandrel link as well as to simplify the process of assembling the mandrel. Examples of mandrel links composed of multiple sections, with each connecting face perpendicular to the longitudinal axis of the mandrel, include U.S. Pat. No. 2,776,697 to Zerlaut and U.S. Pat. No. 3,408,850 to Maier et al. Similarly, U.S. Pat. No. 2,916,077 to Fuchs, Jr., U.S. Pat. No. 3,315,516 to Sassak, U.S. Pat. No. 4,635,464 to McGuire, Sr. et al, and U.S. Pat. No. 3,190,106 to Spates describe mandrel links comprised of two longitudinally facing halves.
Use of longitudinally arrayed link sections facilitates assembly and disassembly. However, if the mechanism for connecting the two link halves does not result in an exact placement of the two halves vis a vis each other, problems result. As the tube is bent the mandrel is subjected to large forces. These forces are exerted on the device in different directions. Thus, the two link halves are likely to be subjected to forces which result in longitudinal displacement, as one half of the link is subjected to larger forces than the other. Not only is this situation detrimental to supporting the interior of the tube as it is bent, the mandrel itself is subjected to rapid wear and eventual failure. The narrow neck portion of a link between the ball and socket ends is particularly susceptible to such forces, possibly resulting in a break at that neck.
It is desirable that the mandrel be prone to resuming its original shape, by some method of detent, so that the same mandrel may be inserted in additional straight tubes to facilitate bending. Particular detent mechanisms known in the prior art are taught in U.S. Pat. No. 3,286,503 to Garrett, U.S. Pat. No. 3,750,455 to Stange et al., U.S. Pat. No. 4,475,375 to Hill, and U.S. Pat. No. 4,315,423 to McGuire. As the mandrel is repeatedly used, known detent mechanisms tend to wear out as a spring used in the mechanism is stretched away from its original neutral position. Eventually, such detent mechanisms may become ineffective, so that the mandrel does not resume its original shape.
While each of the mandrel designs taught in the prior art is useful for its intended purpose, the repetitive forces applied during the process of bending a tube tend to cause wear in the mandrel link and decrease the effectiveness of the detent mechanism. Bending forces can play havoc with mandrels in other ways as well. For ease of assembly and disassembly, mandrel links are frequently constructed of two facing components. As a tube surrounding a mandrel is bent, the forces bending the tube may also result in the mandrel link components being pushed away from each other. A mandrel design is needed which will have a significantly longer useful life, despite the impact of such forces.